A power supply selector can select a power supply and perform switching among multiple power supplies, and the selector is widely used in various kinds of battery backup power management systems, such as a battery charger management chip, a battery backup memory chip, a battery backup microprocessor, and a network processor chip. Taking a lithium battery charger management chip as an example, two power supplies are usually connected to a charger management circuit with a single lithium battery or multiple lithium batteries connected in series: one is an external power supply, and the other is a battery power supply. When the external power supply is on, the external power supply provides energy for the load; and when the external power supply is off, the battery provides energy for the load. Multiple power supplies are usually connected to a charger management circuit with multiple lithium batteries connected in parallel: one is an external power supply, and the others are battery power supplies. Therefore, in various kinds of lithium battery charger management circuits, it is necessary to use a power supply selection circuit to select a power supply from multiple power supplies to supply power for the battery charger management circuits. Generally, the power supply selected by the power supply selection circuit is the supply with the highest voltage in the multiple power supplies. Such a circuit can have two basic functions. The first function is to compare voltages of the multiple power supplies, so as to select one power supply with the highest voltage; and the second function is to transmit the selected power supply efficiently, so as to prevent current from flowing between the power supplies. Currently, two solutions for implementing a power supply selection circuit are used in the field: Solution 1: a power supply selection circuit with a diode structure; and Solution 2: a power supply selection circuit with a switch transistor structure. FIG. 1 shows a two-power supply selection circuit with a diode structure in the prior art. In the circuit, positive ends of two diodes are connected to each power supply respectively, and negative ends of the two diodes are connected together for output. With the circuit, the diode with the highest power supply voltage is turned on, and the other one of diodes is not turned on, so power supply is selected. Meanwhile, such a structure also can prevent current from flowing from a power supply with a high voltage level to a power supply with a low voltage level.
In addition to ordinary diodes, some improved structures are also adopted to implement a multi-power supply selection circuit. For example, Schottky diodes can be employed to replace the ordinary diodes, or transistors can be connected to form diodes which are employed to replace the ordinary diodes. However, no matter what diode structure is adopted, the turned-on diode has a certain voltage drop. The voltage drop of the ordinary diodes and the diode-connected metal oxide semiconductor (MOS) transistors is relatively large, which is 0.5-0.6 V, and the voltage drop of the Schottky diodes is relatively small, which is 0.2-0.3 V. Such a voltage drop is unfavorable, particularly for a low-voltage memory system.
A power supply selection circuit with a switch transistor structure proposed in the prior art is shown in FIG. 2, which can reduce the influence of the voltage drop on the circuit in low voltage applications. The design difficulty of such a structure is how to prevent current flowing between power supplies. Secondly, how to select the type of a switch transistor also needs to be taken into account. A bipolar or MOS transistor may be selected as a discrete switch. The principle is as follows. A selection circuit 21 makes a selection between two candidate power supplies, and signals generated by the selection circuit 21 are used to control the status of each n-channel metal oxide semiconductor (NMOS) switch 22. The power supply voltage output by the selection circuit can be raised to a certain voltage value through an internal charge pump (CP) 23, then the power supply voltage passes through a drive circuit 24, and the output signal is a drive signal of a gate of the NMOS switch. Because a threshold voltage will be lost when the NMOS switch transfers the high voltage level, the raised voltage value should be at least one threshold voltage higher than the highest power supply voltage. Such a design can increase the overdrive voltage of the switch transistor, reduce the on-resistance of the switch transistor, and decrease voltage transmission loss. However, the CP is introduced into the system and meanwhile the system also needs to be equipped with a clock circuit for driving the CP, so that not only the complexity of the circuit is increased but also the power consumption of the system is increased.
The power supply selection to output an accurate power supply voltage without affecting the performance such as the power consumption of the system challenges the design of the power supply selection circuit.